Gas streams containing condensable components, such as sulfur dioxide or various organic vapors, arise from numerous industrial and commercial processes. Venting such gases to the atmosphere wastes resources and causes pollution problems. Industries throughout the world are, therefore, under increasing pressure to clean up waste gas emissions. A widely used treatment method is condensation. The idea is to cool and/or compress the gas beyond the dewpoint of the condensable constituent. A portion of the condensable component will then condense out and can be drawn off in liquid form for reuse or disposal. The degree of removal that can be achieved in this way will depend on the initial concentration, the boiling point of the condensable, and the operating conditions of the process. Problems encountered in such processes are 1) low concentration of the condensable component in the stream, and/or low boiling point, so that the dew point is difficult to reach, and 2) need for regular defrosting. Compressing the gas stream above about 10-15 atmospheres requires large energy consumption and costs increase rapidly in proportion to compressor capacity. If the gas has to be cooled below 0.degree. C., then ice formation in the condenser from water vapor entrained in the feed vapor may occur. Even if the gas stream is pre-dried, taking it down to cryogenic temperatures will again be a costly, energy intensive procedure. Many streams are currently too dilute for recovery by condensation under practical temperature and pressure conditions. Even where favorable operating conditions are possible, 20% or more of the condensable component may be left in the non-condensed bleed gas from the condenser.
Cryogenic condensation and compression/condensation units have been in widespread use for many years. Condensation is a valuable method of waste treatment and pollution control. Nevertheless there remains a longstanding need to improve condensation technology. Recent evidence concerning the adverse environmental effects of halogenated hydrocarbons and chlorofluorocarbons (CFCs) has dramatically intensified that need.
Combinations of membrane separation and condensation are known, including those described in copending application Ser. No. 07/649,305, now U.S. Pat. No. 5,089,033, U.S. Pat. No. 4,994094 and German patent application DE 38 24400 A1. Typically, the membrane separation step is performed first if the feed to be treated has a low condensable component concentration; the condensation step is performed first if the feed to be treated has a high condensable component concentration. Thus, both components of the process can operate in their more efficient ranges; the condenser on a stream that can more readily be brought to saturation and the membrane unit on a stream where a purified residue can be achieved at low stage cut, using modest membrane area. To perform the condensation step first on a dilute stream has been considered undesirable, because the condensation step then requires a higher pressure, a lower temperature, or both, than would be needed if the condensation step followed the membrane separation step. In the extreme case, if the raw feed stream to be treated were so dilute that it could not be brought to saturation under available pressure and temperature conditions, so that no condensation could take place, to attempt to perform the condensation step first would appear perverse.